Since I could not find the culprit after spending hours watching them I remembered someone on the forum using lipstick on the nest entrance to mark who goes in and out... so I did this and the only two are the parents. This after two days, so I have to assume one of them is the problem. I think its the male though as the hen I see go in often to feed (she is also the most brightly marked with the lipstick) the male not so often and I did not see him feed.
So over the weekend no-one was pecked and I thought my interventions of higher boxes etc had worked.
By Sat afternoon one chick was not being fed. I could see why. Its extensive head injury was not allowing it to beg properly, so I took it out and tried the soc's. They were not interested and I hand fed it. Everyone else in the nest was ok.
The handfed I euthanized the next day. He looked to have had a stroke actually, but it was probably just the head injury. One whole side was none functional and he kept falling over and onto his back. Anyhow poor critter is now in the big blue sky which was for the better.
I went to check the nest early this morning and

Getting fed up with this. I will be taking "dad" out today. I may try move the remaining two (as yet unpecked) youngsters with their mum and put them in a cage alone, but that will be a last resort.
I am so going to breed them as individuals once I build my breeding cages.